I think what you are looking for is OneBone, which is already purely CLI.
OneBone isn't Busybox ONLY, but has only Elinks, Ytree and Midnight Commander included extra and weighs only 28M . Check out the link at http://puppylinux.org/wikka/Puplets, to get all the details. If you need to strip THAT further, perhaps the details will at least let you see what sort of requirements are needed.
B

This is an updated take on the Onebone concept for a CLI only Puppy. There is a pretty good but pretty basic set of programs here that should make it possible to do most of the normal tasks of using a computer. This is as far as I intend to take the project, although I haven't ruled out creating a few more packages for it. Here's the highlights.

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